Rearrangement Chemistry

Pathway 2 of Scheme 9 corresponds to one of the most interesting developments in the Beckmann rearrangement chemistry. By trapping of the electrophilic intermediate with a nucleophile (Nu ) other than water, an imine derivative 227 is produced that may be used for further transformations. Carbon or heteroatom nucleophiles have been used to trap the nitrilium intermediate. Reducing agents promote the amine formation. More than one nucleophile may be added (for example, two different Grignard reagents can be introduced at the electrophilic carbon atom). Some of the most used transformations are condensed in Scheme 11. 

The Pirrung synthesis is notable for its brevity and clever amalgamation of [2 + 2] photocycloaddition and Wagner-Meerwein rearrangement chemistry Enol ether 757 was reacted with the Grignard rea nt from 5-bromo-2-methyl-l-pentene, subjected to acid hydrolysis, and irradiated to generate the tricycle 738. Wittig olefination of this ketone and treatment with p-toluenesulfonic acid provided racemic isocomene. 

Methylene difluorocyclopropanes are relatively rare and their rearrangement chemistry has been reviewed recently [14]. In addition, electron deficient alkenes such as sesquiterpenoid methylene lactones may be competent substrates. Two crystal structures of compounds prepared in this way were reported recently [15,16]. Other relatively recent methods use dibromodifluoromethane, a relatively inexpensive and liquid precursor. Dolbier and co-workers described a simple zinc-mediated protocol [17], while Balcerzak and Jonczyk described a useful reproducible phase transfer catalysed procedure (Eq. 6) using bromo-form and dibromodifluoromethane [18]. The only problem here appears to be in separating cyclopropane products from alkene starting material (the authors recommend titration with bromine which is not particularly amenable for small scale use). Schlosser and co-workers have also described a mild ylide-based approach using dibromodifluoromethane [19] which reacts particularly well with highly nucleophilic alkenes such as enol ethers [20], and remarkably, with alkynes [21] to afford labile difluorocyclopropenes (Eq. 7). 

The bulk composition of the SAN copolymer can be determined by ultraviolet spectroscopy. Absorbances consistent with conjugated systems such as Figures 13.1 and 13.2 have been observed. Studies usually compare the UV spectra of model systems with the actual absorbances seen in SAN copolymers. The models represent chemically reasonable species based upon the starting monomers, known reactivity ratios, and oxidation and rearrangement chemistry [2]. The data are self-consistent with these criteria however, identification of all chromophores responsible for color formation in SAN copolymers is still work in progress. One source of species with extended conjugation is the cyclization of acrylonitrile triads to form heteroaromatic structures (Figure... 

Cyclopentene derivatives with unsaturated appendages are also available via this general annulation strategy representative examples are presented in Scheme 15. In these several transformations desulfonylation is achieved using base-induced -elimination, via the fluoride-promoted elimination of 3-silyl sulfones, and by means of Brook rearrangement chemistry according to the method of Reich. In summary, a wide variety of complex functionalized cyclopentenes are available using this versatile [4 1]... 

The [1,2] shift reaction is well known in the rearrangement chemistry of terpenoid derivatives15, as exemplified by the recently described rearrangement of the longipinanc derivative 916. As expected, the migrating bond in this example is orientated antiperiplanar to the leaving hydroxy group. 

In unacidified water at 220 °C in the MBR however, the terpenols (1), (2) and (3) rapidly decomposed. The major products and their relative proportions (Table 1), were consistent with those previously reported by others for the carbocationic rearrangement chemistry of derivatives of linalool, nerol, and geraniol under acidic conditions (13,15-16). 

The first question to be posed in discussing rearrangement chemistry concerns whether a given step exhibits thermodynamic or kinetic control,... 

A critical discussion of Grignard rearrangements has been published recently (29). For this reason, the present review will concentrate on some selected aspects of organomagnesium rearrangement chemistry, rather than attempting to be comprehensive. The most recent literature will be summarized in some detail, but older results will be used more selectively. 

In an additional variation on the theme of spirocyclization via oxidative rearrangement chemistry, oxidation across the indole 2,3-Jt system has been employed by Martin and coworkers in their approach to the core spirocyclic ring structure of citrinadin A [111]. The key epoxidation/spirocyclization was carried out in diastereoselective fashion using an indole A-chiral auxiliary, (—)-8-phenyl-methol carbamate, to direct the asymmetric event. 

The synthesis depends on some key rearrangement chemistry of verbenone (14.4.1), the oxidation product of the abundant natural product pinene. Verbenone provided 10 of the 20 carbons of the baccatin III ring system and also its chirality. Verbenone (14.4.1) was converted to the chrysanthenone derivative 14.4.3 by photorearrangement of... 

Rearrangements (Chemistry) 2. Organic compounds-Synthesis. I. Rojas, Christian Miguel, editor. 

In the rather less developed field of ADPM rearrangement chemistry, there has been limited opportunity to define the stereochemical features of such processes. One salient observation... 

One of the most challenging molecules for organic electronic structure theory is cyclopropane. It is difficult to understand C-C-C angles of 60° with conventional bonding models. Also, in Section 11.5.14 we examined the unusual reactivity of the cyclopropylcarbinyl cation. Much of the rearrangement chemistry of this cation, as well as its special thermodynamic stability (Chapter 2), can be understood to arise from the uniquely strong donor character of the a bonds in cyclopropane. Here we provide a more detailed description of the bonding in cyclopropane that nicely rationalizes these observations. 

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